>MS would get many personal informations. That I understand. And from those informations MS would be able to advertize their apps with great precision. Is that the only reason justifying those billions?

Oh, I guess there are a lots of possibilities there.

For one, advertisements on the Web site itself. If they don't have any, they can be added. This can be done in such a way as to drive clients nuts (and away), or it can be done more carefully, with targeted ads, etc.

Also, even if such a service is free, paid services can eventually be added. The basic subscription would still be free, but certain "advanced" features would require payments.

Also, I understand Facebook has partnerships with several other companies; i.e., part of the content - all those games, you know - is provided by other companies, who, in turn, can get a profit through several means. And of course pay FaceBook for their "hosting" services.